Crown-Rump Length (CRL) Calculator
Enter a crown-rump length (CRL) measurement from an ultrasound scan to get estimated gestational age in weeks and days, plus an expected due date. You can also work in reverse: enter gestational age to see the expected CRL for that week. Both directions use the Robinson and Fleming corrected equation, the clinical standard for first-trimester dating. Results are most accurate between 6 and 14 weeks.
What is crown-rump length (CRL)?
Crown-rump length is the measurement from the top of a developing fetus's head (the crown) to the bottom of its buttocks (the rump), taken when the fetus is in its natural curled position. It is the standard ultrasound measurement for dating a pregnancy in the first trimester, typically between 6 and 14 weeks of gestation. Before the fetal limbs are clearly distinguishable, CRL gives the most reliable estimate of how far along a pregnancy is. It is measured in millimetres on a high-quality ultrasound image, usually when the fetus is in a neutral position without flexion or extension of the neck.
How gestational age is calculated from CRL
This calculator uses the Robinson and Fleming (1975) corrected equation: GA (days) = 8.052 x sqrt(CRL_mm x 1.037) + 23.73. This formula, along with the closely related Hadlock method, is the most widely cited in obstetric practice and is endorsed by the American College of Obstetricians and Gynecologists (ACOG) and the American Institute of Ultrasound in Medicine (AIUM). First-trimester dating with CRL is accurate to within 3-5 days, which is substantially better than last menstrual period (LMP) dating, which can be off by 1-2 weeks. The reverse calculation - finding the expected CRL for a known gestational age - uses the algebraic inverse of the same equation: CRL_mm = ((GA_days - 23.73) / 8.052)^2 / 1.037.
When CRL dating is most accurate
CRL measurements are most reliable between 7 weeks 0 days and 13 weeks 6 days, the standard first-trimester dating window. Before 7 weeks the embryo may be too small to position accurately, and after 14 weeks the fetus begins to adopt variable postures that make a straight crown-to-rump line harder to define. From 14 weeks onward, sonographers switch to biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) as the preferred biometric markers. The INTERGROWTH-21st Project has developed internationally validated CRL growth standards for populations worldwide, providing an evidence base that extends the Robinson and Fleming equation across diverse populations.
How to use this calculator
Choose your calculation direction from the "Calculate" menu. To find gestational age, select "Gestational age from CRL", enter the CRL measurement from your scan report in millimetres or centimetres, and optionally add the scan date to receive an estimated due date. To find expected CRL, select "Expected CRL from gestational age", enter the completed weeks and any additional days, and add the scan date for a due date estimate. The "Show your work" panel traces each arithmetic step so you can verify the calculation. The reference table below shows typical CRL values for key gestational ages from 6 to nearly 14 weeks.
Normal CRL values by gestational age (6-14 weeks)
| Gestational age | Days | Expected CRL (mm) |
|---|---|---|
| 6w 0d | 42 | 5 |
| 6w 3d | 45 | 6.7 |
| 7w 0d | 49 | 9.5 |
| 7w 3d | 52 | 11.9 |
| 8w 0d | 56 | 15.5 |
| 8w 3d | 59 | 18.5 |
| 9w 0d | 63 | 22.9 |
| 9w 3d | 66 | 26.6 |
| 10w 0d | 70 | 31.8 |
| 10w 3d | 73 | 36.1 |
| 11w 0d | 77 | 42.2 |
| 11w 3d | 80 | 47.1 |
| 12w 0d | 84 | 54 |
| 12w 3d | 87 | 59.5 |
| 13w 0d | 91 | 67.3 |
| 13w 6d | 97 | 79.8 |
Expected crown-rump length at key gestational ages using the Robinson and Fleming corrected equation. Actual measurements vary slightly by fetal position and equipment.
Frequently asked questions
What is a normal CRL at 8 weeks?
At exactly 8 weeks (56 days), the Robinson and Fleming equation gives an expected CRL of approximately 15.5 mm. A few millimetres either way still falls inside the normal dating window: on the published Robinson chart a CRL of 13 mm corresponds to 7 weeks 4 days and 18 mm to 8 weeks 3 days, so small differences reflect normal biological variation and measurement technique rather than a problem.
How accurate is CRL for estimating gestational age?
A first-trimester CRL measurement is accurate to within 3-5 days in the 7-13 week window. This makes it the most accurate non-invasive method of dating a pregnancy. By comparison, last menstrual period dating can be off by 1-2 weeks because of cycle variability. If your CRL-derived date differs from your LMP date by more than one week, most clinical guidelines recommend using the CRL date.
Can I use CRL after 14 weeks?
CRL dating is not recommended beyond 13 weeks 6 days. As the fetus grows larger it begins to flex, extend and turn, making a true crown-to-rump straight-line measurement impossible to obtain consistently. After 14 weeks, gestational age is estimated using the biparietal diameter (BPD), head circumference, abdominal circumference and femur length measured together.
How is the estimated due date calculated from CRL?
The estimated due date (EDD) is worked out in two steps. First, the gestational age in days is subtracted from the scan date to estimate the last menstrual period (LMP). Second, 280 days (40 weeks) are added to the LMP to get the EDD. For example, if the scan on 1 January shows a gestational age of 70 days (10 weeks), the LMP would be estimated as 23 October and the EDD would be around 30 July the following year.
Does a small CRL mean a problem with the pregnancy?
Not necessarily. A CRL slightly below the expected value for the reported gestational age can mean the pregnancy dates are a few days off, the measurement was taken with the fetus in a flexed position, or there is normal biological variation. A significantly small CRL, especially combined with a slow fetal heart rate, may warrant a repeat scan in 1-2 weeks. Only a healthcare provider who can review the full scan can interpret a small CRL in clinical context.
What is the Robinson and Fleming formula?
The Robinson and Fleming equation - GA (days) = 8.052 x sqrt(CRL_mm x 1.037) + 23.73 - was derived in the 1970s from serial ultrasound measurements in normal pregnancies and refined using a 1.037 correction factor for measurement bias. It remains the most widely used CRL-to-gestational-age formula in clinical obstetric practice and is the basis for the tables in most obstetric textbooks.
Sources
- Robinson HP, Fleming JEE. A critical evaluation of sonar crown-rump length measurements. British Journal of Obstetrics and Gynaecology. 1975.
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date.
- BC Women's Hospital / Fraser Health Maternal Fetal Medicine. Gestational Age Estimation by Crown-Rump Length (Robinson & Fleming chart).